Unique Double Pulsar Tests Einstein's Theory

In this World Year of Physics, celebrating 100 years since Albert
Einstein published his three epoch-making papers, astronomers are
using a unique pair of orbiting pulsars to make extremely precise
measurements of gravitational effects predicted 90 years ago by
Einstein, and so far, the famous physicist's predictions are proving
exactly correct.

"This pair of orbiting pulsars, discovered only two years ago, is one
of the best laboratories in the Universe for studying the effects of
Einstein's theory of General Relativity," said Dr. Ingrid Stairs, an
assistant professor at the University of British Columbia (UBC) in
Vancouver, Canada. Stairs presented a report on behalf of an
international research collaboration to the American Astronomical
Society's meeting in Minneapolis, Minnesota.

The double pulsar system, PSR J0737-3039A and B, is 2000 light-years
away in the direction of the constellation Puppis. It was discovered
in 2003 by an international team of astronomers and consists of two
massive, highly compact neutron stars orbiting each other once every
2.4 hours. Both neutron stars emit lighthouse-like beams of radio
waves that are seen as radio "pulses" every time the beams sweep
past the Earth.

An artist's impression of the orbits of the two pulsars
(not to scale), showing the "lighthouse" emission beams. The
underlying grid illustrates the general relativistic warping of
space near the two massive compact stars

While several other systems contain two neutron stars, this is the
only one in which both stars are observed to produce radio pulses.
Pulsar A spins once every 23 milliseconds, while Pulsar B is much
slower, rotating once every 2.8 seconds.

The astronomers have been closely tracking this exciting duo with the
Australia Telescope National Facility's (ATNF) 64-meter Parkes
Telescope, the National Science Foundation's (NSF) 100-meter Robert
C. Byrd Green Bank Telescope (GBT) in West Virginia, and the 76-meter
Lovell telescope at Jodrell Bank Observatory (JBO) in England.

Because the pulsars orbit each other so quickly, a number of
effects of General Relativity (GR) are predicted to be quite large.
In fact, four such effects were measured within a few months of
discovery.

Now, the astronomers say they have measured another very significant
phenomenon -- the stars' orbit is shrinking because of a loss of
energy caused by the emission of gravity waves. The orbit is
currently shrinking by 7 millimeters per day, and this decay will
accelerate in the future. This means, the scientists say, that the
two superdense neutron stars will collide in 85 million years.

"The measured decay of the orbit is exactly what is predicted by GR,
so this is another important victory for Einstein's theory," says
Stairs. A similar measurement of the orbital decay in another
double-neutron-star system earned that system's discoverers, Russell
Hulse and Joseph Taylor, the 1993 Nobel Prize in Physics.

Meanwhile, other GR effects are also starting to crop up. One
important prediction is that the stars should wobble like tops as they
move in the curved space-time of their orbit. This should result in
changes in the observed radio pulse shapes, as telescopes on Earth see
slightly different parts of the irregularly shaped "lighthouse"
beams.

The team now has good evidence for this wobble in the slower B pulsar.
Its radio-wave-emitting region is buffeted by the strong particle
winds from the A pulsar, meaning that B is only observable at certain
times of the orbit. Now the times when it is visible have started to
change, and the pulse shapes also have changed. "While we thought
these variations were likely due to the wobble, there was a lingering
possibility that they could have been due to other GR-induced changes
in the orientation of the orbit," says team member Dr. Marta Burgay of
Cagliari Astronomical Observatory (INAF-OAC). Now, however, the
sensitive GBT observations have revealed small profile variations in
the powerful A pulsar as well.

"The A pulse changes can only be explained by the predicted wobble,
which gives us more confidence that the observed changes in B are also
due to a wobble," says team member Dr. Michael Kramer of JBO.

More observations of this unique pair will be needed to use the observed
wobbles to figure out the full geometry of the system, including the
exact directions of the pulsar spin axes and the sizes of the lighthouse
beams. These ongoing observations will also, in the long term, provide
the most precise test of the predictions of Einstein's theory.

"There is a lot more to be learned from this system," says Dr.
Richard Manchester of the ATNF.